How to Compensate Pressure Changes in Temperature Control

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

Use of linear equation is also reported for this purpose; however, this has a low accuracy and is correct for small variations only. For this particular case study, well known Antoine’s equation is used. The methodology described in the article by Jerald Linslley is used here i.e. vapour pressure (Antoine’s) equation for pure component is applied directly to a mixture and empirical coefficients are determined as if the mixture were some pseudo-pure component. In the current text, Antoine’s equation is linearized and coefficients are estimated using multivariable linear regression data analysis tool of MS Excel 2010.

System Description: There are four distillation columns in one of the refinery units, operating at various pressures. Out of these four columns pressure compensation is provided for two columns only, i.e. Deisobutanizer and Debutanizer, which operate at comparatively lower pressure. These columns are explained in detail below.

Temperature Control in Isobuatne Separation

1. Deisobutanizer: Deisobutanizer (DIB) is a superfractionator that separates isobutane (i-C4) from normal butane (n-C4) plus heavier products. The DIB tower has 80 trays which are required to separate the close-boilingpoint components, isobutane and normal butane. Ideally, it is desired to produce the highest purity isobutane in the distillate product and minimize isobutane losses to the bottoms. DIB has three feeds from various sources. DIB has an air-cooled total condenser and thermosyphon reboiler. Refer figure 1 for schematic of DIB. Operating conditions of Deisobutanizer are mentioned in table 1.

DIB Control Schemes

Pressure control: The tower pressure is controlled by partially flooding tube bundle of an air cooled condenser by operating a butterfly valve located at the condenser outlet. If the overhead pressure increases above the set point, the butterfly valve opens and exposes more heat transfer surface allowing more overhead gas to condense; conversely, if the overhead pressure falls below the set point, the butterfly valve closes, submerging more tubes and allowing less gas to condense, which leads to increase in the pressure.

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